In six pairs of chimeric twin calves, the authors plan to first implant a total artificial heart (TAH), then subsequently remove it and transplant the heart from the control twin into the TAH recipient. This is a joint project between the University of Utah and Dr. Shumway's group at Stanford University. The expertise of Utah in artificial heart research, Palo Alto in transplantation, and the tolerance between chimeric cattle twins, promises that these experiments can be carried out with a high degree of success. The main objectives are: 1. The TAH recipient calf will grow, excercise and maintain similar, if not identical blood cellular and blood chemical profiles, as compared to its control twin. 2. Cardiac transplantation in chimeric twins can be accomplished subsequent to chronic circulatory support with a TAH. 3. The replacement of the TAH by a cardiac transplant will not result in major deviations in the measured parameters. 4. The final analysis of the data accumulated by these proposed studies will demonstrate the feasibility for the potential clinical application of the TAH, in the context of the following three proposed settings: A. For definitive recipients wanting an optimally matched cardiac transplant. B. For post-transplantation recipients undergoing irreversible rejection reactions. C. For post-transplantation recipients that become infected while under heavy immunosuppresion. Many of these infections could become ameliorated with proper antibiotics, with an unsuppressed immunological system with good circulation.